This disclosure relates to gamma ray well logging tools and, more particularly, to gamma-ray-transmissive windows that absorb neutrons in such downhole tools.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions.
A variety of downhole tools may be used to determine the properties of a geological formation surrounding a well. Some downhole tools, known as “neutron-gamma spectroscopy” tools, emit neutrons into the geological formation and detect the spectra of gamma rays that result when the neutrons interact with the elements of the formation. Interactions between the elements of the formation and the neutrons may produce gamma rays in at least two ways: by inelastic scattering and by neutron capture. Inelastic scattering occurs when fast neutrons collide with elements of the formation, which may result in the emission of one or more gamma rays. Neutron capture occurs when lower-energy thermal or epithermal neutrons are captured by the nuclei of elements of the formation, which also may result in the emission of one or more gamma rays. In either case, the various energies of the resulting gamma rays may be detected by gamma ray detectors in the downhole tool to obtain gamma ray spectrum measurements. The spectra of gamma rays obtained at various depths in the well may be used to ascertain a variety of different well properties.
Although many gamma rays are generated through interactions between the emitted neutrons with the elements of the formation, some gamma rays may be generated through interactions of the emitted neutrons with the materials of the downhole tool itself. These gamma rays produce a noise background that may reduce the signal-to-noise ratio (SNR) of the downhole tool spectroscopy measurement. Indeed, neutron interactions with the material of the downhole tool occurring near or within the gamma ray detector itself may substantially increase the amount of unwanted background noise. Since these noise-producing neutron interactions occur close to or inside the detector, the detection probability, even in the presence of a low neutron flux, may be high.
The location of the gamma ray detectors in the downhole tools may further increase the likelihood of neutrons being captured by material near or within the gamma ray detector. Indeed, to enhance the gamma ray spectroscopy signal from interactions of neutrons in the borehole and formation, a relatively thin amount of material may separate the gamma ray detector and the formation to reduce gamma ray scattering and absorption in the downhole tool. To reduce the neutron flux entering the detector or nearby parts of the downhole tool, the downhole tool may be surrounded with a layer of neutron-absorbing material to reduce the tool signal. The materials used to reduce the neutron flux entering the downhole tool, however, may also reduce the transmission of gamma rays into the detectors. It may also be noted that some downhole tools with gamma ray detectors, such as gamma-gamma density tools, natural gamma ray tools, and so forth, may use special gamma ray windows or housings made of low density low Z-materials such as titanium. While these windows may offer good transparency to gamma rays, these windows may also have substantial cross sections for interaction with neutrons and, if used in a neutron tool, may increase, rather than decrease, the neutron-induced noise background signal.